text:
Does the tailplane see a higher or lower airspeed than the wing?
The sweepback angle of the horizontal stabilizer is greater than that of the wing. I assume this is to reduce compressibility effects, but it would only make sense if the tailplane sees a higher airspeed than the wing. ¿Is this the case?

This doesn't make sense, the tailplane and wing are attached to the same structure moving through the air at the same speed, so the airspeed of both has to be the same.

@GdD The flow is obviously perturbed by the wing and the effective velocity that the tail sees is not the same as the wing. This is implicit in the question.

"The sweepback angle of the horizontal stabilizer is greater than that of the wing."
Usually it has to be about 5° higher as the the Wing so the critical Machnumber is higher for the Empennage. You dont wanna lose efficiency due to shock waves on the tails (tails have always to work). There are also rules for the thickness and other parameters of the Empennage. You can find some more of them here.
In basic preliminary design the seen speed of the empennage depends on the position of the empennage, but is usually smaller than for the wing, because of the boundary layer(wake). For more detailed design speeds of the jet flow behind a propeller/engine or other cases can be considered too. So not in all cases the airspeed is lower on the empennage and depends on what you are trying to analyze. But the tails should always be able to stabilize the aircraft.

Thanks. So essentially you're saying that the increased sweepback angle on the horizontal tailplane is due to a lower tolerance for compressibility effects on the empennage due to it's function and not due to a difference in effective velocity. I understood that correctly?

Yes, that is correct.
But as always there are also "exceptions" e.g. the horizontal tail of the F18. For the YF17(prototype of the F18) the horizontal tail sweep was also initially higher than the sweep angle of the wing. The Horizontal Tail sweep of the protoype was increased further after realising the control authority was not high enough for starts on aircraft carriers. By increasing the sweep the center of pressure is moving backwards increasing the leverarm without other major changes of the configuration.

@steve, the more important factor is that a higher sweep tail is more tolerant to the angle of attack changes: its critical AoA will be higher than for the wing. It is not uncommon to see a swept tail with a straight wing for very subsonic aircraft (e.g. turboprops).


text:
Is the L-39 Albatros able to go above 40,000 ft in steep flight?
The L-39's service ceiling is 36,100 ft / 11 km but would it be able to easily reach and surpass 40,000 ft MSL if flying a parabola?

I doubt it. The L-39 is a relatively docile training aircraft that some operators have used for ground attack, but the thrust to empty weight ratio is a pitiful 0.49. Put some fuel and a pilot in it and it gets worse. Many refer to it as an aircraft for fighter pilot "wannabe's" with not enough money or experience to fly a more powerful warbird.

@JuanJimenez Good to know. Thanks.

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text:
Why did the SR-71 Blackbird have a light on its top?
What's the reason a military/reconaissance plane would have a light on top of its roof? The SR-71 was the highest-flying airbreathing plane, after all. There was little danger from any collision from above.

It's hard to see a black plane on black tarmac... Planes operate on the ground too. There's also a lot of airspace up to cruise altitude.

@RonBeyer But lights could have been put on its sides or elsewhere.

@Giovanni On its way to altitude, any plane can potentially bump into a higher flying plane from below, no matter the service altitude. Hence, the lights on the top.

@DohnJoe That doesn't explain why it should have a light on its roof, not on the sides where they can be seen from all directions.

@Giovanni It probably has those too.  Most planes have lights at the wingtips, top, bottom, and back at least (in some cases the same light(s) may fill multiple roles).  I assume they can be turned off when running stealth operations.

Plane was flying so high so needed to avoid collisions with UFO and on the top there is no danger of detection by enemies on the ground or lower

@Giovanni: with side-lights and a top-light; it is very easy to assess the planes attitude in darkness from above. Without the top-light, this would still be possible, but probably harder.

That is an anti-collision light as described in the flight manual:

Anti-Collision/Fuselage Lights and Switch
Two combination retractable anti-collision
and fuselage lights are located at the top and
bottom of the fuselage, near the middle. The
lights are controlled by a three-position
toggle switch on the pilot's lighting panel.
The switch positions are: ANTI COLLISION
(forward), FUS (aft), and OFF (center). In
ANTI COLLISION, the lights extend,
illuminate red, and rotate at 45 rpm (which
produces 90 flashes per minute).

(SR-71 Blackbird Flight Manual)
The US AIM 4-3-23 recommends the following (see this answer for more details):

In addition, aircraft equipped with an anti-collision light system are required to operate that light system during all types of operations (day and night).

The idea is that the ground crew and other aircraft on the ground are alerted that the engines are running (or about to be started) or that the aircraft is moving (or about to move). The ground crew can see the lower light (bottom of fuselage), while other aircraft can see the upper one (top of fuselage).
In flight, the SR-71 would switch to FUS or OFF mode:

In FUS (fuselage), the lights are retracted
and illuminate white. The lights are rectracted
and off when the switch is in OFF.

(SR-71 Blackbird Flight Manual)

So was the SR-71 violating any FARs by not operating the anti-collision light with which it was equipped, in US airspace?  Hmmm-- another ASE question there?  Someone should ask it--

@quietflyer The AIM is not regulatory (see this answer). Not sure if the FARs require anti-collision lights, and even if they do, military might be exempt.

@quietflyer: How much jurisdiction does the FAA have over military aircraft?

@mins -- I guess that answers that doesn't it -- !!

@quiet flyer: I'd also assume there'd be a difference between "normal" operations - training, ferrying stuff around, &c - where the military would more or less (certain "hot dog" pilots excepted) comply with civilian practice for safety, and actual military operations.  So the SR-71 would have lights on leaving & returning to base, but not while doing the actual spying.

IIRC when flying above FL600, one is outside the scope of regulation so... I may be very wrong here though... however, general rule in military aviation (the jurisdiction I'm familiar with) is that there's always the "or as commanded" statement. So minimum alt is as per regulations or as commanded, speed limitation 250kts below 10000ft or as commanded. Military pilots always fly according to a mission plan commanded by their superiors, and the mission plan may override pretty much any civilian rule there is, especially in cases where national security is directly involved.

@Jpe61 Above FL600 is not outside the scope of regulation. It just isn't controlled airspace. It's Class E. You don't have to be IFR or have a clearance from ATC, but you do have to follow the FARs... if you're a civilian aircraft, which the SR-71 is not. Of course, if you're a civilian, you will need to be IFR and have a clearance to get to FL600 in the first place, unless you entered the NAS above that level or were dropped off of some air/space craft flying above it.

During missions the SR-71 had to be refuelled air to air many times. These missions took place also at night, so visual cues were absolutely necessary.
Also the mission obviously were often flown  lower and slower in "civillian" airspace for departure and approach legs, so lights were there for general safety also.
At high altitudes they were of course not necessary.

This could, I suppose answer the "In FUS (fuselage), the lights are retracted and illuminate white. The lights are retracted and off when the switch is in OFF." question. Retracted for aerodynamics and white for visibility. After refueling, a quick flick to OFF and the plane goes dark again and returns to spying.

Refuellings were never done in hostile airspace, but lower visibility may still have been desired. Retracted lights probably did not show as widely around the acft as when they were "out". Speed/drag was not of concern during refuel, as the Blackbird was flying very slowly considering its performance envelope. Fun fact: for the latter part of refuelling, one of the engines was set on afterburner to keep the increasingly heavy plane's speed up. Imagine flying that thing with assymmetric thrust at night, thethered to a tanker... I have the utmost respect for those guys!

"Returns to spying" also indicates (in my head, at least), "returns to airspace where spying would take place". ;)  Also, yeah, only one afterburner in the dark tethered to a flying bomb? Yikes!

@FreeMan totally guessing, but the retracted light might be for while-tethered or during the hookup process?  So to minimise loss of night-sight on the boom operator but still being visible in other directions.

@Criggie:  ANTI COLLISION mode is probably even better for night sight: it flashes red (while extended) in that case, vs. solid white in FUS setting  (while retracted).  IDK whether flashing at 90 BPM or solid would be more useful for boom operators, but red lights are more friendly to night vision; red light doesn't trigger the human eye's reaction to contract the iris.

Iirc the benefit of the red ligh vs. Low light vision actually is that red light does not break up rhodopsin which is necessary for rhe rod cells in the eye to work. The downside of the SR-71's red beacon is/was, that it is a rotating configuration, wich can cause disorientation.

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text:
Correcting uncorrected pressure altitude on recorded data
I have some recorded commercial flight data that contains altitude readings defined as "uncorrected pressure altitude." So I have a sequence of lat/lon/alt/timestamps as the aircraft travels about. My ultimate goal is to convert this altitude to an approximation of a WGS84 altitude (height above the WGS84 ellipsoid). The data is from USA if that makes a difference.
What pieces of information would I need, and what formulas should I use, in order to "correct" the uncorrected pressure altitude? Would correcting the pressure even be helpful for my WGS84 goal? (I'm under the assumption that a corrected pressure would help me get to a MSL altitude, and then there's a conversion to WGS84 altitude).
Any advice?

Do you have date and time of the flight?

Yes, it has many timestamps.

Frame challenge: when aircraft are operating below the transition altitude (which in the USA is 18,000' MSL), ATC radar scopes will correct the reported pressure altitude and display the proper MSL figure (rounded to the nearest 100'). If you are only dealing with a small number of flights, you may be able to submit a FOIA request to the FAA and get the stored track data. I do not think this information is kept for a long time, perhaps only 45 days or so.

Some background: transponders always report altitude in flight levels, that is, they report their altitude assuming the local sea-level pressure is 29.92 inHg (1013 hPa). It is up to the receiving station to correct this altitude according to the true local atmospheric pressure and thereby determine the aircraft's MSL altitude. For more information, see Wikipedia: Flight level, PPRuNe: Converting Mode C flight levels to altitudes (and the linked Aviation Formulary site), and Av.SE: How does one calculate true altitude?.
The equation you need, from the Formulary, is:

Find pressure height (altitude) $H_\text{P}$ (in feet) given indicated height (altitude) $H_\text{I}$ and the altimeter setting $A$ (in inches of mercury):
$$ H_\text{P} = H_\text{I} + 145442.2 \left(1- \left[ \frac{A}{29.92126} \right] ^ {0.190261} \right) $$
or reversed to be more relevant for your question, that is, find the indicated altitude given the pressure altitude:
$$ H_\text{I} = H_\text{P} - 145442.2 \left(1- \left[ \frac{A}{29.92126} \right] ^ {0.190261} \right) $$

Note that this will only get you the indicated MSL altitude; that altitude will still not be truly accurate due to other factors, mainly the fact that the rate of change in air pressure is also dependent on temperature. So there is another equation:

Find true height (altitude) $H_\text{T}$ (in feet) of an aircraft, given indicated calibrated altitude $H_\text{C}$, field height (elevation) $H_\text{F}$ of the station providing the altimeter setting, average deviation $D$ (in Celsius) from standard temperature in the column of air between the aircraft and the reporting station, and air temperature $T$ outside the aircraft:
$$ H_\text{T} = H_\text{C} + D \left(\frac{ H_\text{C} - H_\text{F} }{ 273 + T } \right)  $$

I am not sure what the relationship is between indicated altitude $H_\text{I}$ and calibrated altitude $H_\text{C}$, besides the fact that "calibrated" means the value has been adjusted for equipment discrepancies in the altimeter unit itself (as compared to discrepancies in meteorological conditions). It may be that we assume $H_\text{C} = H_\text{I}$. But in any case your data does not include $D$ or $T$ so the second equation is of little use, and you will have to be content with only knowing the indicated altitude.

To perform this correction after the fact, you will need to locate nearby altimeter settings that were current when the aircraft passed by for each data point in your set.
A google search for "archived METARs" resulted in this Av.SE answer which suggests OGIMET. You will have to search by station identifier rather than lat/long so the process will be quite arduous, I'm afraid. (See skyvector for some hints about which station to search for; the colored dots indicate reporting station locations.)
The piece of information you're looking for is the altimeter setting, which is the letter A followed by four digits. In the US that number represents hundredths of inches of mercury, e.g. A2992 indicates a setting of 29.92 inHg.

As for conversion between MSL and WGS84 elevation, I have no suggestions beyond another google search which led to this GIS.SE question.

Thanks for the input. Is there any kind of website or organization I could contact to try and get some of those "nearby altimeter settings" that hopefully were recorded near the timestamps of interest?

@SittinHawk  How long ago are the flights?

A few weeks old

@SittinHawk see my updated answer.

I would suggest that instead of nearby altimeter settings, get ahold of an isobar chart for the area on that day.  Plot your course across it and you will be able to see the pressure gradients so you can interpolate as many points as you like along the route of flight.

Thanks a ton for all the information

The best way to obtain an accurate GPS height estimate requires a static pressure measurement on board the aircraft and an accurate atmosphere model, valid at the time of measurement.
Pressure is measured on board, but often converted to pressure altitude.
If you have static pressure in your dataset, use that. If you have pressure altitude, convert it to pressure using the standard atmosphere model (see ICAO doc 7488)
Now that we have pressure, we take the atmosphere model valid at the time of measurement. This could be obtained from, for example, NOAA or ECMWF.
Within the atmosphere model, typically a GRIB2 grid file, you interpolate the position, time and pressure level and obtain the geopotental height.
The interpolatation requires some thought. Linear interpolation will yield inaccurate results for geopotential height, as the variation of geopotential height with pressure is non-linear but logarithmic.
If you cannot get access to an accurate model, or can live with several hundreds of feet of error in your estimate, you can use a non-standard hydrostatic model that correct for temperature and sea level pressure instead, as is done in this answer.
Now that we have the geopotential height, we can convert to geometric height, using the formulas from ICAO DOC 7488.
The resulting geometric height is measured above mean sea level (AMSL). GPS output is typically with give as height above WGS84 ellipsoid (HAE). To convert between AMSL and HAE, you can use EGM2008.


text:
Trimmed unstable aircraft
I am wondering which conditions are possible and what they mean:

Aircraft is statically stable in pitch and there is a trim point (for a specific surface deflection)

--> So in this situation my aircraft will always return (in case of a disturbance) to the trimmed condition in "free flight mode" , right? d_cm/d_alpha < 0 --> stability criteria

Aircraft is unstable

If I have an unstable aircraft, d_cm/d_alpha > 0 . But to my understanding, there should be still one point (one angle of attack) where it is stable , so a trimmed condition should be still possible. Because I should always be able to make something (like an aircraft) stable by using/moving the right control surfaces to a certain level so that there is no moment acting on the aircraft. This should be the concept of how combat fighters fly? Unstable in the whole but really agile with a fast control algorithm?
Can you confirm me this?
Thank you
Lucas

From a physics standpoint, the term "unstable" means that, if a system is displaced from its equilibrium point, it will experience a net force or torque in the same direction as the displacement. The typical way of visualizing that is to imagine a ball on a dome. If you put the ball at the precise peak of the dome, it can balance there indefinitely. But, if the ball is moved even slightly, gravity will tend to pull the ball farther and farther from the peak, until it rolls off the dome entirely.
In other words, just because the ball isn't moving, doesn't mean that its in a stable position.
The same thing is true for an unstable airplane. Yes, it's possible to get it into a configuration where it's "balanced" (for lack of a better term), with no control input required to maintain straight-and-level flight. But that doesn't mean that its stable. Any disturbance -- a gust of wind, passengers moving around the cabin, fuel being burned, etc. -- will move the plane away from that oh-so-carefully-achieved equilibrium, whereupon aerodynamic forces will start to pull it even further, requiring the pilot (or, more likely, the flight computer) to actively correct.

The statically unstable aircraft has its Neutral Point forward of its Center of Gravity.  Since the statically stable orientation requires the Neutral Point to be aft of the Center of Gravity, the statically stable trim point of the statically unstable aircraft is at some angle to the flow going backwards (if the NP is right on the C of G, there is no statically stable trim point).
In other words, the statically unstable airplane continuously wants to switch ends like a shopping cart being pushed backwards.  It's easy to visualize if you take it to the extreme. Imagine the C of G of an airplane is right back in the tail and you send it through the air.  It will immediately flip around so it's going backwards, like a dart you threw tail first.
So a fighter operating with its C of G right on its Neutral Point will have no stable trim point at any AOA forwards or backwards, and if the C of G is aft of the NP, its statically stable trim point will, theoretically at any rate, be somewhere going the wrong way.
In either case, you now have to use software and sensors to designate an artificially determined trim point at a desirable AOA, and use active flight control inputs, running transparently in the background and not noticeable by the pilot, to hold the body at that AOA as if that was a natural statically stable position.
It's as if you hired someone walking along behind you to hold your shopping cart straight for you as you pulled it along backwards, only allowing it to change direction as they saw you trying to turn it.
You could call that person your Artificial Stability Assistant. If that person lets go, you will have your hands full trying to keep the cart from spinning around. If the statically unstable fighter's multiple redundant artificial stability systems all fail, you'll simply be ejecting.


text:
How does the 'efficiency' of a cyclorotor compare to helicopters and fixed wing aircraft?
I recently rediscovered the cyclorotor concept. There is a good example on youtube here. Ignoring the mechanical complexity to an extent how does the 'efficiency' compare to conventional designs?
When I say efficiency I'm not sure what the best measure is. Possibly lift to drag ratio?
I'm interested in:

lifting capacity: how much weight could a cyclorotor lift vs a helicopter or fixed wing design

energy efficiency

Obviously these things depend on factors like size of the airfoil & airspeed so a rough guide would be suffcient.
I also have the same question about aircraft based on the magnus effect which I have posted separately.

One thing is for sure: after more than a century of fligh all together, and the magnus effect, cycloidial rotors and whatnot being of a ripe age also, they are not very efficient when considering practical use. Otherwise they would be used extensively.

Ask yourself what percentage of the cycle is each wing performing optimally.

I think part of the challenge for the cyclorotor was partly materials and partly engineering. It clearly works but has not had the same amount of effort put into it.

"Ask yourself what percentage of the cycle is each wing performing optimally". That is a very good point but presumably it only applies for vertical take ouff, when a blade is not providing lift it can instead contribute thrust in the direction of flight can't it?


text:
Why does EGPWS use a database rather than just using a built-in forward-looking radar system?
EGPWS is a development of GPWS that uses a database together with the plane's GPS location to create a virtual map of the area around the airplane, allowing it to "see" the ground ahead of it instead of just below it.  However, the result is occasionally not accurate since the database of course isn't perfect.  What I'm curious about is this: why couldn't airplanes just use a forward-looking radar instead?
By the way, I already know about line-of-sight and how radars can't see what's behind an obstacle, but I would think that the highest ground the radar picks up in front of it would be the practical highest point of the terrain around the plane, since if it continues climbing at the same rate after passing above the obstacle in front of it, I would expect it to also clear any higher ground beyond it that went undetected by the radar.

Have you considered cost, size, weight, and power in your calculus?

Or the computing resources needed for real-time interpretation of ground radar data? That makes for a very expensive, power-and-maintenance-hungry piece of equipment.

That's actually how first terrain awareness units worked, by using the radioaltimeter which is a radar.

The reason is weight, electrical requirements and cost. A ground scanning radar is a large, heavy, (very) expensive and power hungry piece of equipment. Very few GA airplanes would be able to carry one, and those couldn't without modifications that would cost more than the value of the airplane.
Or you can have a memory card which costs about $20 and weighs a few grams. The problems with inaccuracies often stem from people not updating their databases, with regular updates inaccuracies are relatively rare.


text:
Airflow through axial flow compressor

So I'm reading the Gas Turbine Engine Fundamental, and suddenly come across the airflow passes through one  axial compressor stage. I wonder why the Resultant Rotor Inlet Velocity is pointing downward, while the Rotor Speed vector is pointing upward. The Resultant Rotor Inlet Velocity is the sum of two vectors Air Inlet Velocity and Rotor Speed. The Resultant Rotor Inlet Velocity is supposed to pointing upward right ??
The same thing happen the rotation direction is downward, why would the Resultant vector pointing upward ??

Can anyone explain for me plz ? Its really confuse me to be honest.

The confusion is because the hypotenuse resultant vector  isn’t really the resultant… at least it’s not the resultant of the two other vectors that are depicted.  (Although it IS the resultant airflow across the rotor…)
As Bianfable mentions, the frame of reference shifts in the labeling of the vectors.  The creator of the graphic is mixing and matching vector labels in a way that might make the point for some, but is confusing for others who are steeped in proper vector math.
To my point, in the first diagram, two vectors represent the direction and magnitude of airflow, while one represents the direction and magnitude of the rotor.  The math doesn’t work when you mix units like this!
So, to make the vector units match just flip the direction of the rotor arrow.  Because the direction and magnitude of airflow over the rotor is opposite to its direction of travel.

Thanks for your answer, I finally understand it. I really appreciate your answer !

The rotor speed vector (green vector in second image) is showing the motion of the rotor in the engine's rest frame. What you are interested in however, is the velocity vector of the air in the rotor rest frame. That means you need transform into the rotating frame by subtracting the rotation vector:
$$ \color{red}{\vec{v}_\text{resultant}} = \color{blue}{\vec{v}_\text{air inlet}} - \color{green}{\vec{v}_\text{rotor}} $$
That way, the resultant vector (red in second image) is the incoming velocity vector for the air from the point of view of the rotor blade.

Velocity triangle was also the topic of this previous question.

Thank you so much for your answer I really really appreciate it, I think I'm quite understand my problem. There just one last thing, I wonder why the velocity at the inlet of the rotor has an opposite direction with the velocity vector at the outlet, although they have the same magnitude ? Anyway, thanks for you help !


text:
Airplane accidents that happen during take off and landing
I have just seen some documentary about Munich Air Disaster in 1958. I found that the real reason behind this disaster is related to the aircraft performance with the thick snow runway during take off. So could you help me to find some more similar accidents that happens due to the aircraft bad performance, espically in take off or descent/landing ? It's my pleasure to see yours answers. My english writing skill is quite bad so pardon me if I made any mistake ! Thanks for reading !

Some on SKYbrary.

If you're looking for documentaries, there's an entire TV series called Mayday/Air Disasters.

Are you asking about accidents caused by snow that reduced the aircraft's performance? Your question seems very broad and I'm not sure exactly what you're looking for.

The problem in the accident you mentioned was not bad performance of the plane, but the effect the thick snow on the runway had on the plane. Basically a pilot error, or bad info given to the pilot. Pilot in command is responsible for assuring the conditions are such that the plane can be safely operated. PIC either failed to do so, or was given false information about the condition of the runway.
Some search terms that would give you accidents of a similar type are icing, de-ice / de-icing, runway contamination, insufficient performance, adverse weather, downpour, and the most obvious slush and snow storm.

Similar accident in 2001 in Bremerhaven, Germany: https://www.bfu-web.de/DE/Publikationen/Untersuchungsberichte/2001/Bericht_3X286-0.01.pdf?__blob=publicationFile

A couple of useful, searchable, resources are AVHerald and the https://www.ntsb.gov/Pages/AviationQuery.aspx.  The problem you'll have is working out a query that will only return the sort of answer you want since accident reports mention take-off and landing, even when they haven't always occurred.


text:
What does W13/S3 mean in this METAR report?
Looking at a recent METAR report for ENQA, I see W13/S3 at the end:
    282050Z AUTO 36017KT 9999NDV BKN013/// 13/10 Q1026 W13/S3

Does anyone know what it means? I couldn't find a description of it online.

I looked too, even fed it into an automated decoder and couldn't find anything. I want to say it's west thirteen, south three, but I do not know what they are referencing.

From metar-taf.com:

Information about the sea
Weather stations located at sea can report the temperature of the sea water and the height of the waves.
W12/S8 means that it is sea water 54ºF [12ºC] with heavy waves. The height of the waves is indicated from 0 (light) to 9 (heavy).

I saw that page and must've missed it, thanks for pointing it out!

It should be noted that ENQA is a natural gas platform off the coast of Norway, where such information is quite important.

It's amusing to me that W12 means 54F[12C] and not 12C[54F].

@J: it may be different elsewhere of course, but in the US our ASOS machines measure the temperature in °F to the degree. They convert this to °C with one decimal place of precision, then round to the nearest °C for the actual METAR/SPECI. And then the pilots take that rounded number and convert back to a now-inaccurate °F figure for the passengers...

@randomhead Yes, we understand why people convert units - the point was that 12 means 12, but can also be interpreted as 54 (following conversion) so it seems to make more sense to say W12 = 12C[54F] and not the other way around, ie W12 = 54F[12C].

@J very true. The webpage itself just says that W12=54°F; I had to add the back-conversion myself. As you say, rather amusing on their part.

These values are related to sea state, and originate from a helideck. W13/S3 means:

Sea surface temperature = 13± 0.5°C
Waves are slight, that is between 0.5m and 1.25m

METAR from helidecks, like offshore vessels and fixed platforms, are commonly generated by a helideck monitoring station, using the auto METAR feature. In that case the mention "AUTO" appears in the METAR message, like in your example.

This METAR originates from Troll Alpha, an offshore plateform in North Sea. Helicopters have limits like the vertical motion of a vessel (heave) or offshore platform where they are landing (the helideck operator may define operational limits too).

Aviation agencies recommend providing the sea status in weather reports, including METARs.

Weather agencies have defined how such reporting is done.

Offshore platform helideck, source
Aviation agencies recommendations
In EASA Technical requirements and operational procedures for the provision of meteorological services

AMC2 MET.TR.215(a) Forecasts and other information SPECIFIC INFORMATION FOR HELICOPTER OPERATIONS

(b) Particular mention should be made of the expected surface visibility, [...],  sea  state  and  sea-surface  temperature,  mean  sea-level pressure, and the occurrence and expected occurrence of turbulence and icing.

Weather agencies
From WMO Aerodrome Reports and Forecasts - A Users’ Handbook to the Codes

Other  information  only  in  accordance  with  regional  air  navigation agreements, including:

Sea-surface temperature and the state of the sea or significant wave height;
State of the runway.

and at section Sea-surface temperature and the state of the sea or significant wave height

Information on sea-surface temperature and the state of the sea or significant wave height shall be given using the following group: (WTsTs/SS’) or (WTsTs/HHsHsHs) where:

W is a letter indicator for the temperature of the sea surface;
TsTs is the sea-surface temperature coded or decoded as indicated in Part A, section 7 (page 21);
SS’ is the state of the sea as indicated in Code table 3700 where S is the letter designator and S’ is the state of the water surface;
HHsHsHs is the significant wave height where H is the letter indicator and HsHsHsis the significant wave height in decimetres.

Example:

Sea-surface temperature: 18.7 °C
State of the sea: Moderate
W19/S4

Table 3700 is found in the WMO Manual on Codes:


text:
How do thick and thin airfoils compare in lift generation?
I find myself confused when comparing the lift generation amongst Thick and Thin Airfoils.
A paper to which I have added reference below claims that thin airfoils are better in generating lift because of which birds tend to have thin curved wings.
But an article from the NASA website, claims the opposite. And many other sources as well claim that thick airfoils produce greater lift.
Can someone please elucidate?
Any help is appreciated,Thanks!

On the thin aerofoil, the amount of flow curvature below the wing is comparable to that above it and we might conclude that the overpressure on the underside is just as large as the suction on the upper surface— the two sides contribute almost equally to the lift. In the case of the thick aerofoil, however, there are regions of different senses of curvature below the lower surface. This suggests that there will be areas with suction as well as areas with overpressure. In this case, the lower surface does not contribute much resultant force and we can conclude that thin aerofoils are better at generating lift.

Babinsky, Holger. (2003). How do wings work?. Physics Education. 38. 497. 10.1088/0031-9120/38/6/001.

Maybe different authors talk about different things: Lift and L/D ratio. See: What are Uses of thin airfoil and thick airfoil?

closely related: What is the effect of airfoil thickness on aerodynamic lift?

Lift is mainly a result of AoA and camber. Thickness has a very small effect, so much so that some aerodynamic theories disregard it entirely. It mostly has an effect on drag and it should be relatively high for subsonic aircraft so that the airfoil can operate at a wide range of AoAs without stalling. It should also be high so that the moment of inertia of the cross sections is high and therefore less stress on structural components. And perhaps most obviously, it should be high so that the fuel tanks fit inside the wing.

You can easily answer to this question looking at the famous theory of wing sections by Abbott. You will find that thickness affects lift as it increases up until 0.12 chord length (c). The effect causes the stall angle to occur later, enabling the possibility to have greater lift, but with the inconvenience that you will have a more abrupt stall. In case of 0 cambered symmetric profiles (let's think to the NACA 4-digit series such as NACA from 0008 to 0012, hence with thickness from 0.08 c to 0.12 c) the stall is more abrupt as the stall angle approaches. In case of cambered profiles this happens more gently. After 0.12 c thickness the maximum lift coefficient tends again to go down slightly (as you can see in the picture you can find here).

As said in the Abbott we have to consider that for Reynolds around 2 milion increasing the thickness over 0.12 c can be considered negligible.

Babinsky, Holger (2003) are absolutely correct in pointing out that overpressure underneath the wing will contribute to lift.

However, this is not necessarily related to "thickness", which is more related to strength.

Scale is extremely important in aviation design:  to support its own weight, scaling a sparrow up to a Piper Cub requires a wing that can support several thousand kg (don't forget manuvering G forces). This will no doubt require a proportionally thicker wing$^1$.
As mentioned in the above reference, curvature, or camber, determines lift at a given speed, wing area, Angle of Attack, aspect ratio, and air density.
At first glance, the thin wing would have the advantage of creating over pressure under the wing to enhance lift, but thicker wings in even the largest planes also have this trick up their sleeves by using ... slats and flaps for lower speed flight.  The "thick" wing will also have curvature on its upper surface, which leads to another very important consideration:
Reynolds number.

Reynolds Number = Velocity × Chord/Kinematic Viscosity

Kinematic viscosity for air is 1.46 × 10$^-5$, the units are meters and seconds
This will lead to a better understanding why larger, faster aircraft fly more efficiently using "top lift".  A look at Reynolds number vs Lift/Drag ratios on Airfoiltools will show a marked increase in L/D ratios as Reynolds numbers rise from 10$^3$-10$^4$ typical of birds to 10$^6$-10$^7$ typical of aircraft.
Larger, faster aircraft can streamline the bottom of the wing, adding strength and reducing drag, by relying on more efficient "top lift".
Here, the legend of the "thick wing" was born, exemplified by fighter designs of the Luftstreitkrafte and the Davis wing  years hence.  Thickness and curvature also become relevant as velocity approaches the trans-sonic and super-sonic realms (critical Mach number), where shockwave drag effects favor thinner wings with less camber.
Bird wings are generally too small and slow to take full advantage of the famed "Bernoulli" top lift effect.  One may realize a diving hawk may cross into higher Reynolds numbers, but they solve this issue with ... variable geometry, by folding their wings!

$^1$ early aircraft builders supported their beloved thin
undercambered wings with cables, and by stacking the wings
(biplane), created very strong, light truss structures.
Emphasis on reducing drag would come later with higher
speeds, enabled by more powerful motors.


text:
Can I log Simulated Instrument time with a safety pilot if I need an Instrument Proficiency Check?
Can I log  Simulated Instrument flight time and instrument approaches with a safety pilot if I need an Instrument Proficiency Check?
I am instrument rated but haven't done my 6 Instrument approaches in over 12 months. So I can not file IFR on my own. Can I bring up an instrument-rated safety pilot 'who is not a CFI/CFII' while I sit left seat with foggles on during VFR conditions and shoot approaches? Can I log it as Simulated Instrument flight time and Instrument approaches even if I need an Instrument Proficiency Check?

Yes, can you still log the time even though it will not count toward recurrency?

Will it count toward my simulated IMC column?

Okay, thank you for the help

There is nothing stopping any pilot from shooting a VFR practice approach under the hood with a qualified safety pilot.  And if you do it properly (hood on until minimums), you can log it.
However, if you are already past the grace period, you still must get an IPC before you can file IFR, and that will reset the currency timer anyway.
So, while these practice approaches may help you maintain (or regain) proficiency before doing the IPC, and you may find it personally useful to have a record of that, it has no legal effect on your currency.


text:
How can speed brakes level a plane?
I copy pasted YouTube's transcript at 2 minutes and 22 seconds. Sweeney refers to then Major Kevin Sweeney, retired as Lt. Col.

NARRATOR: Speed breaks are devices
on airplane wings designed to increase drag
during descent and landing.
Sweeney deploys the speed brakes on both wings,
hoping it will level the airplane.
It's a procedure Sweeney remembers from his training.
It works.
Incredibly, the pilots have managed to level the plane.

Does this trick always work? I'm unschooled in aerodynamics or aeronautical science or physics.

With a swept wing design, a "yaw" (nose moving left or right) can "couple" or cause a rolling motion.  A huge aircraft like that KC-135 tanker, rolled on it's side, no longer has the wings supporting it against gravity.  It will drop like a stone, leading to excess speed, which generally tears the plane apart.  They were in a lot of trouble.
Losing the 2 left engines with power still on the 2 right engines caused a yaw-roll to the left.  As the plane started to sharply descend, the pilot most likely (not show in video), reduced throttle to idle.  Now the 2 remaining engines caused more drag on the right side, resulting in a yaw-roll to the right.
It must be understood that any aerodynamic asymmetry at 500 knots will have very strong effects.  They were rolling very hard in both directions.
Deploying the speed brakes increased drag on both wings, increasing the tendency not to yaw.  We can hear (then) Major Sweeney say "I have lateral control" (can control yawing).  This greatly helps ease the task of rolling the plane upright, which then allows the wing to support the plane and pull out of the dive.
Losing 2 engines may have caused the initial upset, or they could have been torn loose by the violent manuevering.  We'd have to check the maintenance logs there.  One account states they flew into turbulence from another aircraft.

does this trick always work?

Airbrakes (including parachutes) do tend to help straighten the flight path by increasing drag behind the center of gravity.  They are also used to help recover from spins when other control inputs are inadequate.

The crew could also extend the landing gear to even out the drag and get the plane to slow down, as was done during TWA Flight 841's upset. Speedbrakes (aka flight spoilers) blow down at high speed (I'm not sure if it is the same on the 707, but they blow down on the 727) so lowering the gear might be more effective.


text:
Map with names of all active airports for Polar Routes?
Google yielded a map at https://nordregio.org/maps/airports-in-the-arctic-2019/, but it doesn't list any airport codes or names. Any that do?
I am curious how many such airport can assist Polar Route commercial airliners that must crash land in the Arctic. Ugolny Airport - Wikipedia

Owing to its geographic location, its long, concrete-reinforced, heavy load-bearing runway, as well as its modern terminal with jet bridges, the airport is well-suited and well-situated for emergency diversion at roughly the midpoint of the northern route trans-Pacific routes.

On 2 July 2013, a Korean Air Boeing 777-300 performing a scheduled flight from [hicago to Seoul made an emergency landing at Ugolny after one of its engines quit in flight.[3]
On the morning of 5 March 2019, an Air China Boeing 777-300ER performing a scheduled flight from Beijing to Los Angeles made an emergency landing due to a fire alarm later confirmed false. All of the 10 evacuation slides deployed.

I am curious how many such airport can assist Polar Route commercial airliners that must crash land in the Arctic

Which airports are suitable?
The question is more difficult to answer than one might think. First the required length for the runway depends on the type of aircraft and its mass. Small regional aircraft like Airbus A220-100 would take off with less than 5,000 ft and  6,000 ft is considered sufficient for most aircraft. Larger aircraft like Airbus 380, Boeing 747 and Boeing 777 may require more than 8,000 ft. As takeoff requires more runway than landing, it could land on a shorter runway but would then need to take off empty or slightly loaded.
Airports with adequate runways are not so numerous in the Arctic area. In addition the temperatures found at the poles pose a serious threat to humans. Regulations require to have a plan for accommodation and evacuation if no air solution is available to leave the airport within a given time.
But the good news is polar routes are operated under ETOPS which allow to select diversion airports at some distance from the planned route, it's enough the airport is within the reach of the ETOPS time. For example for ETOPS 180, the airport can be within a 3 hours range, and for ETOPS 330, the radius is extended to 5 hours and 30 minutes, which in practical means no constraint in the choice.
The selection of alternate airports involves evaluating candidates for different numbers of passengers, different aircraft, and different periods in the year. Aircraft manufacturers provide their customers with studies about airport capabilities, for example Boeing did say they assessed some new airports for new routes to Asian destinations:

Source
You won't find detailled information online, the results of such studies are sold to customers.
Arctic area airports

Map with names of all active airports for Polar Routes?

Alternate airports have not be within the Arctic area, thanks to the ETOPS standards, but the list of Arctic area airports you're interested in is actually not difficult to compute. They are the ones located past the Arctic circle (66.5°) and with good capabilities in term of runway length.

Google yielded a map, but it doesn't list any airport codes or names. Any that do?

This map shows many mall airports not suitable for airliners, and not suitable for accomodation.
We can create a map for airports matching a given runway length, say 5,000 ft, using OurAirports lists of runways:

Airports beyond the Arctic circle with a runway (any surface) longer than 5,000 ft

The corresponding list is:
$$
 \begin{array} {|c|c|c|c|c|c|l|l|}
\hline
Airport&Lat&Lon&Runways&Length&Surface&Municipality&Airport Name\\
\hline
CYLT&82.5178&-62.2806&05T/23T&5500&GVL&Alert&Alert Airport\\
ENSB&78.2461&15.4656&10/28&7608&ASP&Longyearbyen&Svalbard Airport, Longyear\\
BGTL&76.5312&-68.7032&8/26&9997&ASPH&Thule&Thule Air Base\\
BGTL&76.5312&-68.7032&08T/26T&9997&ASP&Thule&Thule Air Base\\
CYRB&74.7169&-94.9694&17T/35T&6500&GVL&Resolute Bay&Resolute Bay Airport\\
UOHH&71.9781&102.4910&6/24&8872&CON&Khatanga&Khatanga Airport\\
UEST&71.6977&128.9030&3/21&9845&CON&Tiksi&Tiksi Airport\\
PABR&71.2854&-156.7660&6/24&6500&ASP&Utqiaġvik&Wiley Post Will Rogers Memorial Airport\\
USDA&71.2192&72.0522&4/22&8858&CON&Sabetta&Sabetta International Airport\\
PASC&70.1947&-148.4650&5/23&6500&ASPH-G&Deadhorse&Deadhorse Airport\\
ENNA&70.0688&24.9735&16/34&9147&ASP&Lakselv&Lakselv Airport, Banak\\
ENAT&69.9761&23.3717&11/29&7165&ASP&Alta&Alta Airport\\
UHMP&69.7833&170.5970&17/35&8202&CON&Apapelgino&Pevek Airport\\
ENKR&69.7258&29.8913&5/23&6939&ASP&Kirkenes&Kirkenes Airport, Høybuktmoen\\
ENTC&69.6833&18.9189&18/36&7848&ASP&Tromsø&Tromsø Airport, Langnes\\
UOOO&69.3111&87.3322&1/19&9255&CON&Norilsk&Norilsk-Alykel Airport\\
ENAN&69.2925&16.1442&14/32&8097&ASP&Andenes&Andøya Airport, Andenes\\
ENAN&69.2925&16.1442&3/21&5486&ASP&Andenes&Andøya Airport, Andenes\\
ENDU&69.0558&18.5404&10/28&8015&ASP&Målselv&Bardufoss Airport\\
ULMM&68.7817&32.7508&13/31&8202&CON&Murmansk&Murmansk Airport\\
CYUX&68.7761&-81.2425&12T/30T&5410&GVL&Hall Beach&Hall Beach Airport\\
EFIV&68.6073&27.4053&4/22&8199&ASP&Ivalo&Ivalo Airport\\
ENEV&68.4913&16.6781&17/35&9236&ASP&Evenes&Harstad/Narvik Airport, Evenes\\
EFET&68.3626&23.4243&3/21&6565&ASP&Enontekio&Enontekio Airport\\
CYEV&68.3042&-133.4830&6/24&6000&ASP&Inuvik&Inuvik Mike Zubko Airport\\
ESNQ&67.8220&20.3368&3/21&8209&ASP&Kiruna&Kiruna Airport\\
CYCO&67.8167&-115.1440&12T/30T&5500&GVL&Kugluktuk&Kugluktuk Airport\\
EFKT&67.7010&24.8468&16/34&8202&ASP&Kittilä&Kittilä Airport\\
UESK&67.4805&153.7364&1/19&5906&concrete&Srednekolymsk&Srednekolymsk Airport\\
ENBO&67.2692&14.3653&7/25&11136&CON&Bodø&Bodø Airport\\
ESUP&67.2456&23.0689&11/29&7552&ASP&&Pajala Airport\\
ESNG&67.1324&20.8146&12/30&5623&ASP&Gällivare&Gällivare Airport\\
BGSF&67.0122&-50.7116&9/27&9219&ASP&Kangerlussuaq&Kangerlussuaq Airport\\
PABT&66.9139&-151.5290&1/19&5190&GVL&Bettles&Bettles Airport\\
PAOT&66.8847&-162.5990&9/27&5900&ASPH-G&Kotzebue&Ralph Wien Memorial Airport\\
USDD&66.5908&66.6110&4/22&8917&CON&Salekhard&Salekhard Airport\\
PFYU&66.5715&-145.2500&4/22&5810&GRVL-G&Fort Yukon&Fort Yukon Airport\\
EFRO&66.5648&25.8304&3/21&9849&ASP&Rovaniemi&Rovaniemi Airport\\
\hline
\end{array}
$$


text:
Why is this airport SUV following a commercial jet that just landed on the runway?

YouTube

This SUV is flashing yellow lights at the top.

Because if it pulled out ahead of the jet, it would get run over :-)

Does this answer your question? How is a runway inspection carried out?

If it were a takeoff instead of a landing, I'd say it's the last scene from a 90's rom-com...

That's highway patrol performing a traffic stop.  It's probably Will Smith or Tommy Lee Jones behind the wheel.


text:
Safer to stop over in Seattle or Vancouver, and avoid Polar Routes?
My mom works in New York. She survived an airplane crash when younger. Assume cost and travel time are no problems. She prefers paying HER own money and spending MORE time on a stop over flight in Seattle or Vancouver, before flying on to Asia. Her new job may require flying to Seoul, Tokyo, Hong Kong, Singapore on commercial airlines.  Besides, she likes stopping over in Seattle or Vancouver. She wants to retire there and loves their seafood. She assumes flights from Vancouver or Seattle to Asia fly a southerner route and closer to civilization than flights starting from NYC. Correct?
She dreads Polar Routes, because if an airliner  crash lands in the Arctic, survival chance is way  lower! Reasons are obvious!!! The frostiness alone can kill you, and Arctic predators like wolves. Unlike Uruguayan Air Force Flight 571, there's no abandoned summer resort 21 km east. You can't just hike across Queen Elizabeth Islands or swim Beaufort Sea or Arctic Ocean to civilization! If all your pilots died, you won't know your location accurately enough for rescuers. Rescuers may not know IN TIME  if your airliner crashed. Even if they did, they may not find you IN TIME! Rescuers
are too far away!

@4less If you'd like to see the routes taken by airliners in real time check out www.flightradar24.com  Click on the airplane icon and it'll tell you what it is, where it took off from and where it is going.

Statistically speaking, airliners do not have dual engine failures that require a forced landing in the Arctic.
Twin-engine aircraft that fly over water or remote lands are required to comply with ETOPS regulations, which are extremely stringent about maintenance to ensure safety.
From Wikipedia:

ETOPS is an acronym for Extended-range Twin-engine Operations
Performance Standards – a special part of flight rules for one-engine
inoperative flight conditions. The International Civil Aviation
Organization (ICAO) coined the acronym for Extended Twin Operations
for twin-engine aircraft operation further than one hour from a
diversion airport at the one-engine inoperative cruise speed, over
water or remote lands, on routes previously restricted to three- and
four-engine aircraft.

ETOPS twin jet operations began in the mid 1980s and have become extremely common since the late 1990s (thanks to the Boeing 777's ETOPS proving flights which extended ETOPS limits).
Even though airliners are extremely safe, statistically speaking, there is more risk in taking two flights instead of one due to the extra takeoff and landing.

According to Airbus, less than 10% of aircraft accidents happen in the cruise portion of the flight. More than half of accidents happen during landing. You mother is taking the more dangerous option by having more landings than necessary for the trip.
However... I don't think telling her this is going to help her. Commercial aviation is extremely safe, and if having an extra landing eases her fear and allows her to fly, then I would just leave her in her ignorance.

"I would just leave her in her ignorance." I don't want to "leave her in her ignorance"!!! She's open minded, but she just needs some statistics to be persuaded. thanks!

@4less, the moral is, even though two flights are slightly more dangerous than one, the risk is still so low compared to many other normal daily activities that what she feels may outweigh the risk difference. Just tasting the local seafood and stretching their legs is a common (and justifiabe) reason for people to make an extra stopover.

There was a lot of background, but your actual question was: Do aircraft flying to Asia from New York fly farther North than aircraft flying from Seattle / Vancouver, correct? The answer to that is "yes". They both fly more-or-less great-circle routes if they can (based on weather and politics), and the great-circle from NYC to (e.g.) NRT is farther North than SEA-NRT. I will note that even that more-Southerly route lies over the Aleutians, the water there is mighty cold, and the weather there is usually terrible. I would personally prefer to be stuck on winter ice than winter ocean, but you do you.
The Northern route also flies over Russia, with whom we once again are having increasingly strained relations.
If you truly want to avoid the Arctic, you pretty much need to route through Hawaii.

Should be sufficient to route via LAX or SFX; both of those stay (a little) south of the Aleutians on the way to Tokyo, Seoul, or Hong Kong.

"A little south of the Aleutians" is still "die in a few minutes" cold water and not at all closer to civilization.

@Mike Over the Aleutians you can divert to Dutch Harbor, Adak or even Eareckson Air Station if you are an airliner with an emergency.
Approaching the islands you can divert to a number of fields in Alaska.

Yes, the "real" answer would involve computing something like amount of time out of glide distance from an appropriate airport. But the point of most people is that, in the modern era, the maximum latitude of your flight is not at all the biggest factor in its actual safety. If Mom has a (totally understandable) specific fear, she can work around it, but leaving from Seattle isn't that much "warmer", just different. If she likes Seattle, it doesn't hurt to stop there, and it does avoid the 14.5 hour flight that is JFK-NRT.

@JetCityMatt all of which are closed for weather related scenarios a lot of the year, and often have very minimal conditions when they are open.


text:
1. Do commercial airliners turn off vents during boarding? 2. To save money?
BugsArePeopleToo comments on Scientists call for travel ‘code red’ over Covid variant found in southern Africa

But airplanes are still turning off their vents during the boarding process, to save money.

Is this asservation, which can be subdivided into two premises, true?

To quote the reddit comment, "But airplanes are still turning off their vents during the boarding process, to save money."
That sentence has no meaning because airliners don't have "vents", they have air conditioning systems, recirculation fans and pressurization outflow valves (which are always open on the ground).
Depending on the equipment present at the gate at a particualr airport pressurized air can be hooked up at the gate so that there is airflow to the air conditioning system.

@JetCityMatt, of course airliners have vents, there's one at every seat!  A vent is just the opening where airflow from the air conditioning system enters the cabin.

@MichaelHall - that may be what they are referring to, but even then "airplanes are still turning off their vents" doesn't make sense - the vents I have seen so far are purely manual, so the only way to turn them off is to literally turn them by hand.

I suspect (@JetCityMatt et al.) that "vents" is being - badly - used as an abbreviation for "ventilation (systems)". But this generous interpretation of the language probably shouldn't extend to the claims

@rob74, you are right, and if you want to split hairs over literal interpretations you could also argue that “airplanes” can’t “turn off” their own vents, it would require a person turning each one by hand to close them.  But really, it doesn’t take a big stretch of the imagination to realize that the OP is referring to the stoppage of airflow through the vents, (the piece that pax see) via the pilot securing a valve within the system.

The sentence from the linked reddit post that caused the person asking the question to ask the question (word-for-word) made no sense to me when I read it. I was telling the person asking the question that the statement from reddit made no sense. He came here to confirm or disprove what the person on reddit said, which didn't really make any sense. It wasn't really his question (so I wasn't critiquing him- which I was clear about). He was just fact checking someone else's statement (which made no sense TO ME at the time I read it).

@JetCityMatt, it seems you could be letting your advanced systems knowledge get in the way of understanding how a lay passenger might phrase a question about airflow into the cabin.

@Michael Hall, yes, I think you are right.

Do commercial airliners turn off vents during boarding?
No. First, as @rob74 highlighted, overhead vents cannot be remotely closed. To stop air from being refreshed, air-conditioners must be shut off, which is possible. However they are not.
Air is circulated for the purpose of regulating temperature and humidity within the cabin. If air weren't supplied, temperature in the empty cabin would be the same than outside and would increase above acceptable limits with passengers boarding. Air circulation, heating/cooling, relative humidity control, and pressurization are a single system based on air-conditioning packs. This system also regulates temperature in the cargo, in the electronics bays, in the cockpit and around the instruments and electronic devices behind the cockpit panels.
Packs (air-conditioning equipments) are fed with air from compressors. In flight this is bleed air from engine compressors, when engines are off, this is air from the auxiliary power unit (APU), which is a small turbine engine producing no thrust. Some recent aircraft have electric compressors to not use bleed air.

Air flow in the cabin, adapted from source
To save money?
While compressed air and electricity on the ground can be delivered by the APU, operators often prefer to use preconditioned air (PCA) and electricity delivered by airport units. This is a cheaper solution, which doesn't involve aircraft systems and reduces the need for maintenance.
PCA units can be individual units located on jet bridges, centralized industrial units accessed using ground outlets at gates, or mobile PCA units:

Image sources: left, right, bottom
PCA units size increases over time with aircraft size, they are easy to spot with their yellow pipes.
IATA recommendation regarding AC related to COVID
IATA, the union of air transport operators, themselves insists in measures to ensure a good circulation of air, prior and during the boarding and disembarkation:

For those aircraft with air conditioning, run the air conditioning packs (with bleed air provided by the APU) or supply air via external Pre-Conditioned Air (PCA) source at least 10 minutes prior to the boarding process, throughout boarding and during disembarkation. Avoid operations without the air-conditioning.

More: Guidance for Flight Operations During and Post Pandemic

Without cooled air, the temperature inside the cabin would warm up above the outside temperature if you have a lot of warm, breathing people in the cabin. Even on a winter day, without airflow, a full cabin quickly becomes hot & stuffy. An empty cabin is a different case.

Those mobile units are huge! Amazing to think that all that equipment is carried around on the aircraft, as well. Of course, a big portion of that mobile PCA is an engine running a compressor and, well, those are kinda built into the engines that make the plane fly, but still, that's a lot of equipment.

@FreeMan: Also note there is a PCA unit delivering air at 2°C, followed by a cooling unit to reach -9°C. This cooling unit is specific  to wide bodies in hot places, to accelerate cooling.

I did see the daisy-chain. and the second hose, presumably from a second pair.

I guess the origin of this myth is that once the AC is switched over to the airplane's systems (or from the APU to the main engines), it usually increases in intensity - although that probably also depends on the method used before the engines are started...

In the schematic, where does the ground unit tie into the airplane system?

@OrganicMarble: I detailed the pneumatic system of the A320 in this answer. From it this schematics, Air from ground supply enters a mixing and distribution unit like air extracted from the cargo holds by recirculation fans. Actual design varies by aircraft. The principle schematics in the current answer is not entirely representative of an actual pneumatic system.

Thanks for the informative links!


text:
What is the difference between buffeting and wake galloping?
I am interested in flow-induced vibration and confused between buffeting and wake galloping. As far as I have studied, both phenomena are the vibration of an object with another object in front of it. But I still don't understand what the difference between these two phenomena is.
Thank you

Related: What is the difference between buffeting and fluttering?

"Galloping" seems to be synonymous with the aviation term "flutter", which is a (potentially disasterous) reinforced harmonic oscillation of an aircraft component, such as an aileron.  "Buffeting" is usually applied to a the felt effects of turbulent airflow on a surface, such as a pre-stall warning buffet.
Turbulent flow can be random or rhythmic.  It is the rhythmic pattern, in conjunction with the natural vibrating frequency of the trailing object, that can cause the "flutter".
The "galloping" term seems to be used more frequently in other engineering pursuits, such as power line transmission cables, bridges, and off-shore moorings because the frequency of a very long cable will be much slower than the "buzz" of an aircraft flutter.

See this article on the famous "Galloping" suspension bridge.


text:
What is the tapered cone on the B747 CF6 engine inlet cowling?
B747 and B767s are supposed to be using exact same GE CF6 engine, and as such their components are interchangeable including inlet cowl as they show to be effective in the IPC.
However some of the 747 cowls have this tapered cone on them:

That I have never seen on a 767-300s cowls. What is the function of that cone and what is it called?

The title question and the body don't match...

@Bianfable its for the GE CF6 engines

@RonBeyer I will modify it and let me know if its fits better


text:
Can a jet-engine aircraft fly all the way to its service ceiling at its maximum takeoff weight?
I am trying to understand how high an aircraft can get to at its maximum takeoff weight (minus whatever fuel is consumed in getting to that altitude). Can it make it all the way to its service ceiling? Or is there a payload/altitude tradeoff beyond a certain level? If there is such a trade-off, at what altitude does it start to occur? Meaning what is the highest a plane can go at its maximum takeoff weight? Is it the cruising altitude? If not, what is it called and where would one find it for specific planes (thinking of KC-135R, A330 MRTT and Pegasus KC-46).
Apologies for throwing so many questions at once.

Those are all fairly complex calculations depending on atmosphere conditions and will be very different from one aircraft to the next. You're going to have to narrow it down some.

@TomMcW Right. For instance, the KC-135R has an MTOW of 322,500lb and a service ceiling of 50,000 ft. I am trying to understand if it can make it to 50,000 ft at ~320,000 lbs (minus the fuel burned) or does one have to be traded-off for the other? If there indeed is a tradeoff between maximum weight and service ceiling, then could you help me understand how high can it get at its MTOW? Now I understand that this would depend on atmospheric conditions, so please feel free to assume a scenario if that helps.

The problem is that this would differ between types. Some may have enough engine power to reach the ceiling at MTOW and others not. Someone would have to have access to the particular numbers for each type. Typical ariliners on long haul flights have to do a step climb as fuel is burned. But at what point they have to do that would require calculations specific to airframe and engine stats. Since the KC-135 is based on the 707 I doubt it could, but it might have different engines. You'd have to find someone with those numbers. There is not a generalized answer to your question.

The absolute MTOW is a limit imposed by structural considerations, not altitude limitations. At every point on earth, a max allowable TO weight must be established by the pilots based on runway length, pressure altitude, climb performance etc.
The service ceiling can be reached at structural MTOW: it is defined as the altitude where the maximum rate of climb = 0.5 m/s (100 ft/min). At lower weight, the aeroplane simply needs less thrust to reach cruise speed. It will be cruising at an altitude below the service ceiling.

@Koyovis What about the MTOW set by the manufacturers then? Also, is the maximum rate of climb independent of the weight of the aircraft then? Shouldn't the rate of climb go up (all else being equal) if a plane is lighter?

@757toga Have amended the phrasing, thx. Reaching the service ceiling at MTOW will be very sub-optimal for fuel consumption and severely limit range, however add more thrust and the plane will climb.


text:
Could anyone provide me with the performance charts for Boeing KC-135R Stratotanker?
I am looking for the Long Range Cruise Maximum Operating Altitude performance against the weight of the plane, all engines operating. Would be grateful if someone could point me the way.

I could only find this, and it doesn't answer your question, it just says the info has to be calculated. https://static.e-publishing.af.mil/production/1/af_a3/publication/afman11-2kc-135v3/afman11-2kc-135v3.pdf


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Can gliders access restricted airspace?
If I recall, in America there were several "Prohibited Access" airspaces and others required permission, via radio, to enter in a typical aircraft. But, what if I'm gliding over a restricted airspace? (Say, over the Capitol?). Would I be shot down (or fined, sued, etc.)?

Since gliders have no engines, they are often permitted to operate under special agreements called Letters of Authorization, which permit them to operate where other aircraft may not. These agreements may permit them to fly VFR in Class A airspace to capture mountain wave lift, through TFRs, through Restricted airspace, depending on the terms of the agreement with the FAA.
In particular, in the airspace you mention near the Capitol, there is a TFR Waiver in force which permits gliders to operate within the Capitol TFRs.  This Waiver supersedes the TFR rules and the associated NOTAMs. Furthermore, it permits gliders to even thermal inside the TFR if necessary for safety of flight.

The waiver info you linked to is for a TFR which is in force around Camp David during presidential retreats, not the U.S. Capitol.

That TFR is part of a set of TFRs commonly known as the Capitol TFRs

Sure, but it doesn't permit flight over the Capitol as in the example the OP gave.  I'd be surprised if there were any waivers that allowed gliders into the FRZ.

I addressed his question regarding 'gliding over restricted airspace'.

It doesn't matter what category of aircraft that you are flying, the rules for restricted and prohibited airspace applies the same unless there is a specific exemption in a NOTAM for that, which I haven't seen before.

Although this answer is correct, all it basically says is "the rules are the rules." All glider pilots know, however, that Letters of Authorization and Waivers provide gliders across the US access to airspace that would are not normally be accessible to other aircraft. Please see my answer below for a fuller explaination.

As Inafziger has already said, there is no difference between a glider and a conventional airplane.
However it is possible to fly with both types of airplanes into all kinds of airspaces, if ATC granted the entry. In 99 per cent of the times they will require a transponder though, which only a few gliders have. (I think most aerobatic gliders have one)

I wouldn't assume that aircraft can fly into any airspace just because ATC said it was okay.  See: Can you bust a TFR while operating IFR?.  You are correct though that you can often enter restricted airspace if you  follow their terms.

If the controller explicitly grants you the right to fly through this airspace, I would be very surprised if you can actually be charged for that. But I would not want to try flying into a TFR either way.

Well, asking won´t hurt - I´ve come across several cases where after a nice chat with ATC crossing of restricted airspace was granted even in a glider without transponder.

@Force Take a look at the link in the question that ibreferenced in my previous comment.  It can (and has) happened.

I think @Force wouldn't be with ATC with his glider unless he wants to make sure if an airspace is hot or if he can pass through. If you get the info by ATC that you can make your way through this airspace you are on the safe side. It's something different if you just set up a flight following and did not ask for the particular airspace. They will probably warn you prior violating an airspce or TFR but that's not for sure. Remember that it's a voluntary service and most of the controlleers are busy.

It goes beyond just 'ATC granted the entry.' Many airports with glider operations have "Letters of Authorization," with the FAA that allow gliders to operation in airspace around the gliderport that they wouldn't normally allowed to enter. Here's one for Williams: http://www.williamssoaring.com/pilot-info-wave.html and for Minden/Pyramid: http://www.airsailing.org/downloads/2011/ER-ZOA_NCT_PASCO_Pyramid_Minden_Glider_Area_20101029_ESV.pdf

@Falk the operating times and controlling agenccy for Restricted airspace is on the so-called "tabs" of Sectional charts. So long as you're operating outside those times, you're in good shape to fly through them.

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Where does the Oxygen for the emergency gas masks come from?
Normal commercial jets drop gas masks when cabin pressure drops, and supply oxygen, but, where does that oxygen come from? Do commercial jets store oxygen? If they do, isn't pressurised oxygen very flammable and too hazardous to carry?

Perhaps I'm just splitting hairs, but a "gas mask" does not drop. A gas mask is essentially nothing more than an air filter (whose fit to the face forms a seal intending to prevent air from coming in from the sides). The mask that drops on aircraft is not a filter, it's a source of oxygen giving you something to breath.

On commercial aircraft the oxygen supply of the masks in the cabin comes either from a chemical oxygen generator or a centralized oxygen bottle system. The flight deck is always equipped with pressurized oxygen masks.
The chemical oxygen generators generate lots of heat when they are activated. When normally placed in their overhead compartments the heat is slowly dissipated. However, oxygens generators are hazardous when canisters are not in their appropriate place. The ValuJet Flight 592 crash into the Everglades on 11 May 1996 was caused by a fire after improperly packed oxygen generators in the cargo hold were accidentally activated.
Pressurized oxygen bottles also are hazardous. Not only because the oxygen can turn a small fire into a blaze but if the high pressure bottles explode they cause substantial damage. On
25 July 2008 an oxygen bottle installed in a Qantas Boeing 747 ruptured and propelled itself upward through the floor into the cabin.

It's maybe useful to note that they're actually called "oxygen candles" in the industry... they are essentially burning chemicals which produce oxygen as the waste gas.

Just to add to your answer, on my commercial aircraft we do have a central oxygen bottle, which is very common with smaller jets.

@LNafziger You're right, and after some thinking, I realized that it is not only the case on smaller jets. The B747-400 has also a bottled oxygen system, and I remember an accident related to that as well :-)

Wikipedia has an excellent article on the subject:
http://en.wikipedia.org/wiki/Emergency_oxygen_system
In some cases the oxygen masks are connected to a central oxygen supply (or more accurately, a  supply of breathing gas, containing a mix of oxygen and other gas). The oxygen can also be generated using a chemical device, one for each oxygen mask or seat row.
Side note: contrary to what many people believe, oxygen is not required during normal flight. The outside air at high altitude still contains the same percentage of oxygen (21%) as on the ground. The outside air is pressurized and fed into the fuselage, allowing normal breathing.

Airliners do not carry tanks of oxygen for the "rubber jungle" when the ox masks are deployed.  The oxygen is created by chemical reaction within canisters stored in the overhead that are not activated until the rubber jungle drops down.
This chemical reaction does create a lot of heat.  That's what caused the Valuejet fire and crash in the Florida everglades a few years ago.  The canisters were being shipped and were not clearly marked as hazardous cargo, and other safety steps were missed that may have inerted the canisters.
Given the many millions probably billions of canister-hours that these devices have been used over the years, I would conclude they are safe.  Given the disaster that happened when they were handled improperly for shipping, I would also say the airlines cannot lower their guard on shipping these apparently harmless canisters at any time, ever.

""Airliners do not carry tanks of oxygen for the "rubber jungle"" - some of them do.

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How does a student pilot learn the process of pre-flighting a Robinson R22?
I've watched a few youtube videos on pre-flighting a Robinson R22 helicopter, and I'm always impressed by the way the pilot methodically does the external checks and the internal pre-start, start-up and warm up.
For me, there's a big gap: knowing what to check (because the checklist says so), and being able to identify it (put your finger on it / point to it), and knowing how to check it.
How does a student pilot learn the process?
How long does it take to master the process?
Assuming being able to pre-flight is a pre-requisite for flying solo, and a private pilot has one lesson per week, and there are say 20 1-hour lessons before solo is even considered, then 20 opportunities to learn such a complex set of processes does not seem enough repetition and re-enforcement somehow, and the risk of stuffing it all up seems very high.

There's an enormous difference between watching a YouTube video and actually walking around a machine you're about to go flying in.

On your first lesson (of flying any new aircraft), you will spend time with your instructor to go through each and every part on the aircraft. What takes 5 minutes to do on video, may take the instructor some 2 hours to explain it to you.

"The checklist says to check X, but I can't find it on the aircraft. Ah well, how important can it be? I'll just skip that item. Surely someone checked it earlier and it was fine." Said no responsible pilot ever.

I can only speak to airplanes, but when I was learning my first lesson had about an hour just going over the various parts of the aircraft. After that my instructor would preflight while I watched/repeated and asked questions for a few lessons then I would take over. Aside from how they look, aircraft aren't (purposely) overly complicated. It helps to have a knowledgeable CFI though, for example on my 177 I wouldn't have known to check the fuel drain retaining clips (it isn't on the checklist) because they've caused accidents jamming ailerons, but my CFI went over it with me.

@RonBeyer "fuel drain retaining clips (it isn't on the checklist) because they've caused accidents jamming ailerons" That seems odd to me, to be honest. I suppose they may or may not be in the checklists in the POH(?), but there's nothing preventing the use of a different, more thorough checklist if you know the manufacturer's one doesn't cover some particular, relevant item, as long as everything from the manufacturer's checklist is on it and especially for something like a preflight, where you have ample time.

@MichaelKjörling They aren't on the checklists because the retainers were added later as an AD (and did not include any POH modifications). It is very true though that you can add as much as you want to your checklist as long as it covers at least as much as the POH does. However I still wouldn't have known to add it to my checklist if nobody had shown it to me in the first place.

I have a little over 100 hrs in an R22, and the POH section 4 explains what to check and what to look for, that is the best and safest way to perform a preflight inspection. If you're not sure what your looking at ask the Instructor. I would never rely on memory for a preflight Inspection.

Welcome to aviation.SE!

I have never flown the R-22 but I am rated in RH and ASEL. I can answer specifically about the Schweizer (Sikorski) 300C/ CB/ CBi helicopter.
1) How does one learn the preflight? From your instructor. During each lesson the first part of touching the aircraft should be the preflight. Legally speaking, that is after touching the log books/ registration/ weight and balance/ etc. to make sure that it is potentially airworthy in the first place. First you watch them do the inspection, then you do it under supervision, then they watch you from a distance, then the instructor finishes up whatever they are working on last while you preflight it solo (with the best instructors doing the preflight themselves before you even start).
2) How long does it take? For the Schweizer it took me probably the first 10-12 lessons since I was initially not able to fly frequently enough to remember most of what I learned the last lesson and I was working 80+ hours per week at the time. Going 1-2 months between lessons is very bad for retaining new information. Had I worked less and flown more it probably would have taken half of that for me to get comfortable doing the preflight myself.
The items on the checklist are really not that hard when you understand the function of the parts you are checking. When you check control rod integrity you know what they do. You follow them from the cyclic and collective through the cabin, up the mast, through the mixer arms, and to the swash plate. You check the swash plate. You follow up the mast to the control rods attaching from the swash plate to the rotor blades themselves. Other than the climbing around the aircraft it probably took me longer to type that than to actually do it for a preflight.


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CO-meter indicates 10ppm of Carbon Monoxide when I turn on Cabin heat - should I be concerned?
Our Cessna 172P is equipped with a digital CO-meter rather than the traditional Quantum eye carbon monoxide detector.
Today I was flying along in winter conditions, and naturally used cabin heat. During the flight, I heard some occasional short beeps, and couldn't figure out what it was. I looked at the CO-meter which has a red alarm light, but that light was definitely off.
On the ground, after shutting down the engine, I heard another short beep, and it definitely came from the CO-meter! The digital readout was showing 10ppm.
During post-flight, with the door open, the readout dropped to 0 ppm.
I routinely check the CO-meter battery during preflight, so I'm pretty sure the beep is not a reminder for a dead battery.
According to these sources: http://www.coheadquarters.com/ZerotoMillion1.htm , http://library.thinkquest.org/10121/textonly/exposure.htm and wikipedia 10ppm is nothing to be concerned about with exposure over a two-hour flight.
Does anyone else have a digital CO-meter and can tell if a reading of 10ppm is normal with cabin heat?
Or should I be concerned our exhaust or heat exchanger is faulty?

Hi Phillip, what kind of CO meter is it?  The manual for it should have information about when you need to be concerned....

Thanks for all answers. I told maintenance to check at the last 100hr inspection. Since then, I haven't had that beeping anymore.

The short answer is you should be concerned.
Although 10PPM of carbon monoxide doesn't pose an immediate health risk for short term exposure the fact that the level is rising when you turn on the cabin heat & falling when you ventilate the cabin (by opening the door) indicates you may have an exhaust leak, and those tend to get worse over time.
You should have your shop open the heat muff & inspect the exhaust system for cracks/leaks before your next flight (or if that isn't possible avoid (or limit) use of cabin heat & defrost & monitor the CO meter as part of your scan until the exhaust system can be checked).
You should also check your CO meter: electronic CO meters have a finite life, just like chemical spot detectors (though they generally last several years) - your detector's manual will specify a replacement interval. As the sensors age they may give inaccurate readings.

A note about ground operations - Electronic CO detectors are sensitive enough to occasionally generate transient alarms during ground operations due to exhaust gasses entering the cabin. This can happen during summer or winter.
As long as the levels are low and the alarm clears when you're moving (taxi or during climb-out) this is probably not a problem, but you should know what is "normal" for your aircraft.

Even though voretaq7's answer is correct, I just want to add that there is also a chance that the sensor is broken.
You should know that the sensor is made of a ceramic plate that has to be heated in order to get a readout, and then it is cooled down again. Also it has to be calibrated before using, to get an absolute measurement instead of a relative measurement.
Have you ever left the sensor in a very cold/wet or very hot place? Or did you leave it in the aircraft on the ground during a sunny day? The ceramic plate can easily be damaged by exposing it to the conditions mentioned above. You might want to test it in another plane and if it still shows the same output, the chances are high that the sensor is broken.

Indeed, one of the things to be concerned about here is an inaccurate sensor - Both electrochemical and semiconductor/ceramic sensors can fail if "abused" as you describe. In this case the sensor is behaving in a manner consistent with detecting carbon monoxide (it's beeping when cabin heat is on, and during ground ops, and the level drops when the cabin is ventilated), but if the sensor has been abused or is past it's lifespan the reading may be inaccurate, and you can't easily tell if it's reading low or high.


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Does turning off WAAS turn off ADS-B out as well?
I'm a CFII and for training GPS approaches with advanced students, I like to occasionally turn off the WAAS receiver on the 430/750/G1000/etc. when they are not looking, to simulate a WAAS failure and make them notice they need to change their minimums.
Now, ADS-B out depends on a WAAS GPS-receiver. When I intentionally turn off WAAS, does that turn off ADS-B out as well? Would I be technically non-ADS-B-compliant in 2020, when flying the approach without WAAS?

I think ADS-B usually uses a separate GPS receiver, in which case there would be no effect. That may be installation dependent.

@GdD Yes, it is, but on systems with a Garmin GNS430W, the 430W can feed the WAAS signal to a Garmin ADS-B like the GTX345, this is how it is in my aircraft. It also feeds GPS signal to our G5.

How exactly do you disable WAAS on a 430? It’s not something I’ve come across in the manuals or heard about before.

Your ADSB Out device may or may not have an independent position source; if you don't know, find out before doing this again in rule airspace.

This depends on the ADS-B receiver that you have, but if it is still receiving position information, but the WAAS portion is missing, you will get a lower NAC/NIC/SIL in the broadcast message. If you disable the GPS completely, the ADS-B will continue to function however the SIL for position will be the lowest level (likewise for NAC/NIC). The ADS-B will continue to function (as in send messages) but the position reports will be unavailable.
It's important to note that your transponder is still a Mode-C transponder, even if the ADS-B fails (completely or partially). You should inform ATC that you have an ADS-B or GPS failure and they will inform you if you are cleared to continue or not.
Really though as far as this is concerned, creating an actual failure is very much not recommended. If I were you, I'd just say "let's pretend that you have a RAIM message about your WAAS, what are your new minimums?" Creating an actual failure is something like pulling the bulb out of one of the gear indicators. It might be a "teachable" moment at first, but really just creates a distraction that can be done in other ways. What happens if the circuit breaker fails and the weather actually drops? Now you have a real problem...

In fact the transponder will not only be still a Mode C transponder but even a Mode S transponder ‘cause neither of those is depended of ADS-B or ADS-C.

Interpreting your question more broadly along the lines of "would I be non-compliant with the ADS-B Out Rule":
RTCA's DO-260, the standard that defines requirements for 1090ES ADS-B Out and In systems, states the following in section 2.2.3.3.2.12:

The ADS-B Transmitting Subsystem shall terminate broadcast transmissions of the Airborne Position Message when position (latitude/longitude) and altitude data are not available for a period of 60 seconds.
Note: For the Airborne Position Message, altitude data alone is sufficient
  to maintain broadcast of the Message once the Message has been
  initiated. When only altitude data is available, the Airborne Position
  Message continues to be transmitted even after 60 seconds. However, if
  the altitude data is not available for 60 seconds, then the Airborne
  Position Message transmission is terminated and the conditions for
  start-up defined in §2.2.3.3.2.1.2 are necessary to resume the
  transmission of Airborne Position Message.

Similarly, for the velocity messages:

The ADS-B Transmitting Subsystem shall terminate broadcast
  transmissions of the Airborne Velocity Message if input data necessary
  to update the subfields of the Airborne Velocity Message [...] is not available for a period of 2.6 seconds.

Interestingly, however, there are additional messages (e.g. status messages, identification messages, etc.) that will continue to be broadcast even in the absence of usable position information:

The ADS-B Transmitting Subsystem shall not terminate broadcast
  transmissions of Aircraft Identification and Category Message even if
  input data necessary to update the Message is not available.

Comparing this against the text of the ADS-B Out rule, §91.225:

§ 91.225 [...]
(b) After January 1, 2020, and unless otherwise authorized by ATC, no
  person may operate an aircraft below 18,000 feet MSL and in airspace
  described in paragraph (d) of this section unless the aircraft has
  equipment installed that
(1) Meets the performance requirements in [...] TSO-C166b [...]
(2) Meets the requirements of § 91.227.
  [...]
(f) Each person operating an aircraft equipped with ADS-B Out must
  operate this equipment in the transmit mode at all times [...]

So, one could say that you have a properly certified ADS-B transmitter installed, you are operating it in transmit mode, and it is actually broadcasting messages; therefore you are in compliance with the rule.
That said, I am not sure I would risk my CFI ticket on this argument...

Two points -
1. compliance with 91.225 requires performance that meets TSO-C166b - that TSO invokes DO-260B.  Earlier versions of ADS-B Out do not comply.
2. Compliance with 91.227 requires broadcasting (approx.) 18 message elements, including position and velocity.
So, while the OP may be compliant with the equipment requirement of 91.225, he/she would not be compliant with 91.227.


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Are there any aerospace examples of applying a boat tail?
A way to reduce the pressure drag of bluff bodies is to apply a tapering of the tail (so called boat tailing)
I know some (extreme) examples of this method of drag reduction applied to cars:

Source
And I know that this a feature of the Very low drag bullet

Source
I was wondering if there are any aerospace applications?

I think the more difficult question to answer is "are there any aircraft that do not use this principle".  I bet there are, but none spring to mind immediately...

I understand that all aircraft shapes are aerodynamically shaped, but I'm looking for examples where a bluff body has a relatively small region that is tapered, rather than a teardrop shape as whole

Planes with teardrop shape as a whole are very rare, because it is impractical. Anything that should have significant amount of internal space has constant cross-section for most length.

Aerodynamic fairing fitted to the space shuttle for transport aboard NASA's 747 ferry aircraft

Although uncommon it has been used for aerodynamic and stealth reasons on fighters.  An example is the chisel like engine enclosures on the YF-23 aircraft.

This depends on the mission the plane is built to fit. Some planes need the cargo space so the tail is less tapered but most planes have at least some taper. Diamond Aircraft have very tapered tail sections

(source)
Even the older GA designs like the Piper Seminole have a good deal of tail taper

(source)
Larger planes have some tail taper as well however sometimes there is less as there are often systems in the tail like the APU or there is storage space.

(source)

Almost all plane designs are made in wind tunnels and have a tapering empennage. Some notable exceptions which only taper in one dimension are rear-loading cargo planes like the C-130 and the Shorts Sky Van.

Your last two examples there are tapered, but only from one direction, which is actually fairly similar to the car posted in the question.

but it might just as well be tapered like that so the tail doesn't strike the runway on takeoff and landing.

It's more for cargo loading as your pictures illustrate well.

you don't need the taper for loading. its the lack of taper (as I mentioned in my answer) which is for loading

Lack of taper on one side, all taper on the other.


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Did the Space Shuttle boat tail used in the transport configuration save fuel?
During the testing phase as well as during transport of the Space Shuttle on top of a 747, a boat tail was used:

Source
Here I found that this was mainly to reduce the turbulence that would strike the tail of the 747, thus increasing yaw authority.
However, I can also imagine that the boat tail would reduce the base drag of the Space Shuttle, thereby reducing the fuel penalty of carrying the Space shuttle.
Is there any research that shows that the boat tail also reduced the drag, rather than just providing a cleaner flow to the vertical tail?

I don't think the fact that it is the only option removes the necessity to reduce drag?

@Airsick too much drag, and you won't even take off.

@Antzi An empty space shuttle weighs 75t, Boeing 747's MTOW is 333t and empty weight is 162t. There's still a lot of space for fuel so that you are way below MTOW. My guess is that some added drag is not that much a problem.

It's interesting to note that photos of the Buran on the An-225, which has an H tail, don't show any boat tail on the Buran.

@fooot, that's probably because the An-225 was purposely built to transport the Buran. As such, the wake coming from the Buran was taken into account from the beginning. Therefore it has the large H-tail. As there is no rudder to 'protect' using a boat-tail, there is apparently no need for a boat tail.
The 747's were already designed before, and were adapted to carry the Space Shuttle.

Yes, and there are certainly other factors to consider. Just pointing out that if there were a drag reduction reason, the Buran may have used one.

I have always been under the impression that it was mainly installed to protect the engines. However this blurb (and cool photo) claim it did reduce drag.

...reduces both drag and turbulence created by the piggy-backing spacecraft while also protecting the engines


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What are these thermal spots on this infrared image of an F-16?
In this question, Hephaestus Aetnaean posted the following cool movie:

which showed the following still:

As indicated by the red circles (the yellow marking can be ignored) there seem to be a few hotspots visible on the IR image. What are these hotspots? They seem to be rather random, so I'm not sure what they exactly are.

the one near the nose is a heated air data probe (see the video at 0:18)

Sorry for the yellow spot, it was my first attempt to mark the spots, but I didn't found it obvious enough and couldn't be bothered to make a new screen capture :p

The yellow spot is probably the sun reflection (yes I understand you painted it yellow, but its on the canopy where the sun would typically reflect).

The hot spot in the cockpit could be the pilot's cigar.

In this picture from Wikipedia it can be clearly seen that the two along the bottom of the fuselage are both heated air data probes, as Federico pointed out. The probes are heated to prevent ice from forming and blocking air flow to the sensors. The forward one is a pitot probe, and the aft one is a total temperature probe. The AOA sensor can also be seen as a much smaller hot spot above the forward pitot probe.
I agree with Ron Beyer that the top area is probably the sun reflecting off of the canopy. The lower area is probably a sun reflection as well, off of the external fuel tank. You can see in the video that these areas are not always visible, and move as the F-16 flies past, consistent with a reflection.

Good answer but to clarify the forward probe is only the Pitot probe. The static port is further back flush with the aircraft skin. They work together in the same system. The system itself is referred to the Pitot-static system. Just wanted to clarify it.

@Mark thanks, fixed.

I believe what you are seeing is the infrared image picking up heat sources. As the plane flies through the air friction is caused with any object sticking out from the skin of the aircraft. The nose of the aircraft has a Pitot tube which by design sticks out in the airstream. This causes a lot of friction. Additionally the pitot tube is heated to prevent ice from clogging the tube. As for the other spots, I am unsure as to the devices but they appear to be in the airstream as well. Perhaps some are small antennas.


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What is the reason for the strips on the nose of the Embraer E175
Recently I boarded a KLM flight with an Embraer E175, when I noticed the strips on the nose
(see image, marked with the black circle).

Own image
This Embraer E190 also has it:

Image source
Whereas this A320 does not have it, is it an Embraer specific feature? What is it for?

Image source

I believe this is a duplicate of another answer, but they are lightning arresters which conduct electrical charges away from the radome and avionics contained therein to the metal airframe where it can pass through and exit the aircraft without harm.

They're used more to discharge static buildup on the radome than handle lightning strikes.

Well and what is lightning but a large static electric discharge?

Fair enough. But the point is to not have a large static discharge at all